Bandpass filters are widely used in radio communication systems. At microwave frequencies, electrical signals are often guided by transmission lines in the form of rectangular waveguides to minimize losses of the signals. Waveguide filters may be implemented using shunt inductive irises along the waveguide structure forming resonating cavities coupling to one another. Such filter design is well described in the book, "Microwave Filters, Impedance-Matching Networks, and Coupling Structures" by G. Matthaei, L. Young and E. M. T. Jones at pages 450 to 459.
One problem with waveguide filters, especially at lower frequencies, is the size or more specifically the length of the filter, which is a limiting factor when it comes to integrate them in today's compact radio systems. For example a six-cavity filter at 5 GHz is about 12-inches long. This length can be reduced by one-half when superposing every other adjacent cavity to implement a folded structure. These "folded" structures can be machined out of a block of brass or copper, but in some cases, to achieve the required temperature stability, a more stable material has to be used. Machining the filter from a block of INVAR is very difficult and expensive.